The present invention relates to an optical recording medium and, more particularly, to an optical recording medium having a phase change recording film which reversibly changes its phase upon irradiation of light and thereby changes its optical characteristics.
The entire surface of a recording film of a phase change type optical disk is in amorphous state, i.e., in recorded state immediately after the manufacture. Ideally, in this phase change type optical disk, a recording mark as an amorphous state and a background as a crystalline state are formable by overwrite recording immediately after the manufacture.
When a recording film is formed by the conventional method, however, the amorphous state of the recording film immediately after the formation is excessively stable. Therefore, first overwrite recording using an optical disk drive cannot turn the recording film crystalline immediately after the formation. For this reason, the general approach is to crystallize the entire surface of the recording film by using an initial crystallizing apparatus having a high-output laser, before data is recorded in a phase change type optical disk by using an optical disk drive.
Phase change type optical disks as described above are roughly classified into two types: High to Low (to be referred to as HtoL hereinafter) media whose reflectance in amorphous state (recorded state) is lower than in crystalline state (erased state), and Low to High (to be referred to as LtoH hereinafter) media whose reflectance in amorphous state (recorded state) is higher than in crystalline state (erased state).
Since higher reflectance can be obtained in crystalline state rather than in amorphous state in the HtoL media, the reflected light amount cannot be insufficient immediately after the initial crystallization. Accordingly, the HtoL media do not make servo control unstable when overwrite recording is performed by using an optical disk drive immediately after the initial crystallization.
When data is overwritten in the HtoL media immediately after the initial crystallization, the average reflectance lowers because the recording mark formed is amorphous. In this case, however, servo control cannot be unstable. This is because the area of a recorded portion and the area of an unrecorded portion in an overwrite-recorded optical disk are substantially equal, so the average reflectance is about intermediate between the reflectance in crystalline state and the reflectance in amorphous state. That is, in an overwrite-recorded optical disk, a reflected light amount enough for stable servo control is obtained.
In the HtoL media as described above, sufficiently high reflectance is obtained both immediately after the initial crystallization and after the overwrite recording. Hence, servo control cannot be unstable in the HtoL media.
In contrast, in the LtoH media, the reflectance in crystalline state is lower than that in amorphous state. Therefore, a sufficient reflected light amount cannot always be obtained immediately after the initial crystallization. Accordingly, the LtoH media may interfere with servo control for the reason which will be described in detail below.
Recently, recording marks are becoming increasingly finer as the recording density of optical disks increases. Therefore, to obtain a large signal (reflectance change rate) from a fine recording mark, the reflectance in crystalline state is often decreased in the LtoH media.
This will be described below by using a modulation factor. Letting R.sub.a be the reflectance in amorphous state and R.sub.c be the reflectance in crystalline state, the modulation factor is defined as (R.sub.a -R.sub.c)/R.sub.a. This modulation factor is an index of the magnitude of a signal. More specifically, it is possible by decreasing R.sub.c to increase the modulation factor, i.e., obtain a larger signal.
Presently, to realize a recording capacity of 4.7 GB per side of a disk to meet an increasing recording density of, e.g., a DVD-RAM, a track pitch of about 0.59 .mu.m and a bit pitch of about 0.28 .mu.m are under examination (the track pitch of a DVD-ROM is made narrower than that of a DVD-RAM because the former is a read only memory). Under these conditions, when a mark edge recording method is adopted, the C/N ratio (Carrier to Noise Ratio) must be 53 dB or more in order for a reproduction jitter amount to be 8% or less. According to calculations, this means that when R.sub.a is 20% and R.sub.c is 4%, the modulation factor is 80% or more.
Unfortunately, if the reflectance of the recording film immediately after the initial crystallization is set to 4% or less in the LtoH media, no sufficient reflected light amount is obtained when overwrite recording is performed by using an optical disk drive, so servo control becomes unstable. If information is recorded in this state, such inconvenience as the inability to reproduce recorded information takes place.
To eliminate the above inconvenience of the LtoH media, the use of a non-initialization type optical disk is pursued. This "non-initialization type optical disk" means an optical disk capable of overwrite recording using an optical disk driver immediately after the manufacture without requiring any recording film initial crystallization.
In this LtoH non-initialization type optical disk, first overwrite recording is performed for an amorphous recording film immediately after the manufacture, i.e., for a recording film whose entire surface is in recorded state in which the reflectance is high. Therefore, the LtoH non-initialization type optical disk does not presumably interfere with servo control when first overwrite recording is performed.
Although the non-initialization type optical disk has the aforementioned superior characteristic, this disk also has the following problem: when information is recorded in a predetermined track by radiating a laser beam, information recorded in adjacent tracks may be erased, i.e., cross erase may occur. Accordingly, reproduction of recorded information is sometimes difficult in the non-initialization type optical disk.